Gas turbine engines are routinely used in modern day aircraft. In these jet propulsion systems, air is drawn in at the upstream end of the engine and the pressure, density and temperature of the air are raised by a compressor. Just downstream from the compressor, fuel is injected into the air and burned, adding energy to the gaseous fluid. The gaseous mixture then expands and passes through a gas turbine (which drives the compressor) on its way to the exit nozzle. There it emerges into the atmosphere at an extremely high velocity and at a pressure equal to or greater than the atmospheric pressure surrounding the nozzle. Lift and propulsion of the aircraft is thereby achieved.
Current turbine engines are characterized by demanding performance specifications over an incredibly wide variation in temperature. It is not unusual for the engine specifications to require performance at temperatures ranging from subzero up to about 2000.degree. F. Therefore it is necessary that the manufacturing techniques used to form these engines allow very little room for deviation. In particular, the turbine blades which are used within these turbine engines are manufactured to precise specifications, having dimensional tolerances often within a few thousandths of an inch.
Generally, the turbine blades are characterized by a relatively complex shape, having a thin leading edge seam and a wider trailing edge seam. Internally, the turbine blade typically has at least one cavity for cooling purposes. These turbine blades are sometimes cast as a single unit, however in the more complex designs, the turbine blade is formed by joining two matched halves. The turbine blade, whether a single unit or two matched halves, is typically formed from a single crystal of a nickel based superalloy which is characterized by high strength at these demanding temperature extremes.
There have been many methods for joining the two single crystal, matched halves. A particularly common method has been to diffusion braze the two mating halves together. In diffusion brazing, a braze filler alloy is provided between the two base metal turbine blade halves in those regions where the two halves are to be joined. This is typically not only at the leading and trailing edges, but also at the root and tip portions of the blade, as well as the internal cavities within the blade. When heated to the brazing temperature, the braze alloy melts. The braze alloy contains a melting point suppresser, generally boron, which diffuses into the single crystal base metal during the brazing operation. Since the melting point suppresser has been diffused out of the braze filler alloy, the re-melt temperature of the braze alloy will correspondingly increase. Therefore, solidification of the braze alloy will occur at the brazing temperature as the braze alloy is depleted of boron Brazing time and temperature are determined so that the re-melt temperature of the braze alloy (which has been depleted of its melting point suppresser) is the same as the melting point of the single crystal base metal.
A problem is encountered when edges of thin sections of the single crystal nickel based alloys are diffusion brazed together, such as in particular when the leading edges of the matched turbine blades are brazed together. Due to the sluggish rate at which the boron diffuses into the single crystal nickel alloy, high levels of boron exist both in the braze alloy within the joint region and in the surrounding diffusion zone of the base metal after brazing. Because the solutioning and melting point temperatures for the single crystal nickel alloys used for these turbine blades are extremely close, the post brazing solution heat treatment at the solutioning temperature for the single crystal nickel based alloy, may actually cause melting within those regions where high levels of boron exist, such as in the brazed joint and surrounding diffusion zone. This is particularly troublesome since melting may disrupt the single crystal structure of the turbine blade and correspondingly diminish its performance characteristics.
In addition, another problem exists with regard to the joining of these matched turbine blade halves. When proper edge alignment between the matched blade halves is not achieved correctly, an unfilled edge joint will remain after brazing This may also occur with turbine blades which have a brazed on cover sheet for providing transpiration cooling of the blade. In either instance, if proper edge alignment can not be maintained during the joining process, an open seam will exist. This gap, particularly when at the leading edge of a turbine blade, will seriously effect the aerodynamic performance of the blade during operation.
Therefore, what is needed is a method for joining matched, single crystal turbine blade halves which alleviates the shortcomings of the current brazing methods, particularly when joining the thin wall sections of these halves such as at their leading edge. It would be desirable if such a method did not rely on diffusion brazing techniques, so as to alleviate the use of a melting point suppresser and accordingly eliminate the problem of post-brazing re-melt during solution heat treatment of the single crystal, nickel alloy, base metal. It would also be advantageous if such a method were capable of providing a metallurgically compatible material in the joined region as compared to the base metal. Still further, it would be desirable if such a method could completely and uniformly fill the joint between mating blade halves despite any misalignment between those halves.